The present disclosure, in various exemplary embodiments, relates generally to electrophotographic imaging members and, more specifically, to layered photoreceptor structures having a charge transport layer comprising certain terphenyl diamines.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges can be retained on its surface. Upon exposure to light, the charge is dissipated.
An electrostatic latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of the many known means in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity on the conductive substrate. A light image is then projected onto the photoconductive layer. The portions of the layer that are not exposed to light retain their surface charge. After development of the latent image with toner particles to form a toner image, the toner image is usually transferred to a receiving substrate, such as paper.
A photoreceptor usually comprises a supporting substrate, a charge generating layer, and a charge transport layer (“CTL”). For example, in a negative charging system, the photoconductive imaging member may comprise a supporting substrate, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and an optional protective or overcoat layer. In various embodiments, the charge transport layer may be one single layer or may comprise multiple layers having the same or different compositions at the same or different concentrations.
The charge transport layer usually comprises, at a minimum, charge transporting molecules (“CTMs”) dissolved in a polymer binder resin, the layer being substantially non-absorbing in a spectral region of intended use, for example, visible light, while also being active in that the injection of photogenerated charges from the charge generating layer can be accomplished. Further, the charge transport layer allows for the efficient transport of charges to the free surface of the transport layer.
When a charge is generated in the charge generating layer, it should be efficiently injected into the charge transport molecule in the charge transport layer. The charge should also be transported across the charge transport layer in a short time, more specifically in a time period shorter than the time duration between the exposing and developing steps in an imaging device. The transit time across the charge transport layer is determined by the charge carrier mobility in the charge transport layer. The charge carrier mobility is the velocity per unit field and has dimensions of cm2/V sec. The charge carrier mobility is generally a function of the structure of the charge transport molecule, the concentration of the charge transport molecule in the charge transport layer, and the electrically “inactive” binder polymer in which the charge transport molecule is dispersed.
The charge carrier mobility must be high enough to move the charges injected into the charge transport layer during the exposure step across the charge transport layer during the time interval between the exposure step and the development step. To achieve maximum discharge or sensitivity for a fixed exposure, the photoinjected charges must transit the transport layer before the imagewise exposed region of the photoreceptor arrives at the development station. To the extent the carriers are still in transit when the exposed segment of the photoreceptor arrives at the development station, the discharge is reduced and hence the contrast potentials available for development are also reduced. The transit time of charges across the charge transport layer and charge carrier mobility are related to each other by the expression transit time=(transport layer thickness)2/(mobility×applied voltage).
It is known in the art to increase the concentration of the charge transport molecule dissolved or molecularly dispersed in the binder. However, phase separation or crystallization sets an upper limit to the concentration of the transport molecules that can be dispersed in a binder. One way of increasing the solubility of the charge transport molecule is to attach long alkyl groups onto the transport molecules. However, these alkyl groups are “inactive” and do not transport charge. For a given concentration of charge transport molecule, a larger side chain can actually reduce the charge carrier mobility. A second factor that reduces the charge carrier mobility is the dipole content of the charge transport molecule in their side groups as well as that of the binder in which the molecules are dispersed.
One charge transport molecule known in the art is N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD). TPD has a zero-field mobility of about 1.38×10−6 cm2/V sec at a concentration of 40 weight percent in polycarbonate. Zero-field mobility μ0 is the mobility extrapolated down to vanishing fields, i.e., the field E in μ=μ0 exp(βE0.5) is set to zero. In general the field dependence expressed by β is weak.
There continues to be a need for an improved imaging member having a charge transport layer with high carrier charge mobility. Such an imaging member would allow for increases in the speed of imaging devices such as printers and copiers.